6850 is also factory OC at 800Mhz. It would probably blow the doors off of a 7770. So, if they're both factory OC, they're on about the same level as far as enhancements, so this could be considered accurate:
Which means that, if you have the added power to spare, go with the 6850. You wouldn't be disappointed with the 7770, though, as the drivers are still maturing slightly. You'd get a small gain, but probably not one to fully trounce a 6850.